In a typical ozone bleaching process, pulp and an ozone containing gas are fed into an ozone reactor wherein they react to bleach the pulp and produce an exhaust gas. The amount of ozone required by the reactor is dependent on various factors such as in-going pulp brightness, production rate and target K No. or brightness.
Precise application of ozone on pulp is critical for maintaining bleaching performance. The extent of bleaching is directly proportional to the amount of ozone applied. Too little ozone results in a darker than desired pulp, while too much causes over-bleaching and a reduction in pulp strength. Short term variability of the incoming pulp results in rapidly changing optimum ozone application levels, which necessitates rapid flow system response to changes in ozone demand. Therefore, the over-riding control constraint of any ozone bleaching process is to precisely match ozone production and application to the incoming pulp brightness or K No. and tonnage.
A commercially suitable process for controlling the flow of ozone gas into a reactor is disclosed in commonly-assigned, copending U.S. application Ser. No. 07/604,849, now U.S. Pat. No. 5,181,989, to White et al., entitled "Pulp Bleaching Reactor and Method." In that application, an apparatus and method for delignifying and bleaching a lignocellulosic pulp without the use of elemental chlorine is disclosed. The bleaching reactor is a horizontal vessel having a central rotatable shaft which preferably contains paddles, cut and folded screw flights or a ribbon flight, to disperse and advance the pulp particles in a plug flow manner while contacting and mixing the pulp particles with a gaseous bleaching agent such as ozone for substantially uniform bleaching thereof.
After ozone containing gas has reacted in an ozone reactor, it is economically desirable to recycle the spent gas due to the relatively high cost of pure oxygen. When the spent gas is recycled, various contaminants must be removed from the recycle stream to maintain efficiency in the ozone generation process. A system for removing contaminants, including carbon dioxide, from an ozone generation recycle stream is disclosed in commonly-assigned copending U.S. application Ser. No. 07/739,050, to Joseph et al., filed Aug. 1, 1991, entitled, "Process and Apparatus for Conditioning Ozone Gas Recycle Stream in Ozone Pulp Bleaching." In that system, carbon dioxide and other contaminants are removed in amounts sufficient to prevent build-up in the ozone recycle stream of a pulp bleaching process providing the advantage of maintaining a higher ozone generation efficiency and reducing the overall cost of operating such a system.
In a recycling loop of the type described above, exhaust gas containing oxygen, carbon dioxide, water, residual ozone, hydrocarbons and carbon monoxide, is maintained at a predetermined pressure by a compressor. The exhaust gas is then recycled in a gas destruct loop where carbon monoxide and hydrocarbons are removed. The recycled gas is then refrigerated and dried to remove all water. Oxygen is then added to bring the gas to the desired composition and the gas is then fed into ozone generators wherein the oxygen partially reacts to produce a desired percentage of ozone. This ozone containing gas is then fed into the reactors where it bleaches the pulp.
In such an ozone recycle loop, tight control of flow rates, pressures and compositions of the gas recycle stream are necessary for optimum generator and reactor performance. Further, since electricity costs account for a large portion of the ozone production operating costs, maximum efficiency at the ozone generators is desirable. This efficiency, in turn, is a strong function of gas composition and temperature, and to a lesser extent pressure. As discussed in the above-mentioned patent application to Joseph et al., it has been found that a maximum concentration of 9% carbon dioxide (91% oxygen) is commercially feasible in that it allows the ozone generators to operate at an optimum efficiency. Thus, it is desirable to feed gas having a concentration as close to 91% oxygen as possible into the ozone generators and to minimize variation in the composition of this gas entering the ozone generators. Further, should an upset in oxygen composition occur, caused, for example, by a change in reactor demand, it is similarly important to provide a rapid return to set-point after such an upset.
As for the generator pressure, which as mentioned above effects generator efficiency, but to a lesser extent than gas composition, constant pressure at the ozone generators is desirable for two reasons. First, ozone generation efficiency is maximized at a specified pressure. Second, a constant generator pressure ensures constant head to the reactor control valves, simplifying the control action of those valves.
A strategy for controlling the gas recycle loop to meet the above-described goals has been developed wherein spent gas is compressed and then passed through a thermal and catalytic destruct sequence to convert residual ozone, hydrocarbons and carbon monoxide to carbon dioxide and water. The gas is cooled, and a purge stream taken to prevent the accumulation of carbon dioxide. The gas is then refrigerated and dried in an absorption unit to remove all water. Oxygen is then added to bring the gas to its desired composition. The oxygen containing gas then passes to an ozone generator where the oxygen partially reacts to produce a predetermined percentage by weight of ozone. This ozone containing gas then flows to the reactors where it bleaches the pulp.
This control scheme includes a pressure control valve, driven by a controller, which maintains constant pressure at the generators. This system thereby largely solves the problem of maintaining generator pressure.
Further, in this control scheme, the purge rate of carbon dioxide is controlled based on the oxygen composition of the gas entering the ozone generator. The flow rate of added oxygen is based on ozone generator pressure. While it has been found that this strategy keeps the oxygen content of the gas entering the ozone generator within an acceptable range, the response time for return to set-point, i.e., 91% oxygen, after a change in flow rate through the recycle loop is relatively slow and thereby reduces generator efficiency. Thus, when reactor demand changes, the above-described system is slow to re-establish the proper oxygen composition.
Therefore, it is desirable to design a control strategy that provides minimal variation in oxygen composition and a rapid return to the set-point after an upset.